Drilling rigs in deep water have long needed lifting towers capable of lifting heavy items that are moveable on a deck. The ability to move this lifting tower significantly reduces safety hazards on a rig and the enables smoother operation and greater flexibility for various operations.
Blankestijn U.S. Pat. No. 6,171,027, published Jan. 9, 2001, discloses a drilling rig, in particular for the offshore industry, in the form of a Jack-Up platform on which a cantilever is mounted so as to be movable at least substantially horizontally and in a first direction, with the cantilever projecting more or less outside the Jack-Up platform. On the cantilever a drilling derrick is present, which drilling derrick is movable relative to the Jack-Up platform in a second direction, different here from. This drilling derrick is fixedly mounted on the cantilever while the latter is movable relative to the Jack-Up platform in both above directions. However, this invention does not disclose having a tower, such as a single or double hoist tower mounted on the cantilever.
In the drilling rig according to the prior art the cantilever and connected drilling derrick are movable on rails using rollers or sliding plates. These rails are fixed onto the deck or are formed of a three-part housing that workers can trip over as they move along the deck. Accidents of all sorts have occurred with the presence of the rails or three part housings on the deck of the jack up rig and other various other sorts of drilling rigs. Furthermore these rails or housings considerably limit the number of possible uses of the deck areas on which the rails or housings are fixed.
On drilling rigs according to prior art a drilling derrick is fixable mounted on the cantilever. One of the properties of a drilling derrick is the large base needed for stability. This large base determines the minimum width of the cantilever. Another disadvantage of a drilling derrick is the relative inaccessibility of the drill floor due to the large number of structural beams that are in the way. This severely limits the possible number of useful activities that can be carried out on the drill floor and consequently the number of useful activities that a drilling rig can perform.
On the majority of the drilling rigs of prior art the drilling derrick moves in transversal direction on the cantilever, the latter that can only move in longitudinal direction. The cantilever beams of existing platforms limit the transverse movement of the drilling derrick on the cantilever. As a consequence, the drilling pattern is limited to drillings within a small rectangular shape. To obtain a drilling pattern, which is nevertheless acceptable the cantilever should be of a relatively wide design.
Also when a movable drilling derrick has moved over the maximal distance in the transversal direction on the cantilever the cantilever beam on the side to which this movement took place is subjected to a considerably heavier load than the other cantilever beam. Because of such asymmetric loads on the cantilever beams occurring in practice these beams should be of a relatively heavy construction. A further drawback of the conventional rigs is that between drilling rig and cantilever, as well as between cantilever and drilling derrick, flexible connections should be provided for pipes, cable work, etc.
A need has existed for a cantilever with tower that has:                a. A moving system which is simple and more versatile compared to the rails or three part housings        b. One unit sliding bearings;        c. A lower weight        d. A smaller width        e. Increased accessibility and safety of the drill floor        